Conventionally, some proposals are made on retroreflective articles formed of hexagonal cube corner retroreflective elements having excellent retroreflective efficiency and entrance angle characteristics. However, a few techniques disclose what method is preferable for providing excellent observation angle characteristics.
For example, U.S. Pat. No. 1,591,572 (Patent Document 1) by Stimson discloses hexagonal cube corner retroreflective elements. However, there is no description as to what shapes of elements provide excellent entrance angle characteristics, observation angle characteristics, and rotation angle characteristics.
U.S. Pat. No. 3,417,959 (Patent Document 2) and U.S. Pat. No. 3,922,065 (Patent Document 4) by Schultz disclose a method (pin bundling method) of forming a prism at the tip of a metal pin and bundling a number of such pins to form a prism assembly. This method is suitable for producing relatively large prisms, but is not practical when it is necessary to form 2,000/cm2 or more microprisms, for example.
U.S. Pat. No. 3,458,245 (Patent Document 3) by Stanley discloses retroreflective prisms having an obtuse angle and an acute angle alternately arranged in at least two faces, or preferably four faces or more.
U.S. Pat. No. 3,924,929 (Patent Document 5) by Holmen et al. also describes a retroreflective article formed of a repeated pattern of units in which hexagonal prisms are hermetically sealed.
U.S. Pat. No. 4,066,331 (Patent Document 6) by Lindner also describes a retroreflective article in which different hexagonal prisms are arranged in each row.
U.S. Pat. No. 4,073,568 (Patent Document 7) by Heasley also describes a retroreflective article in which one kind of hexagonal prism is arranged in a repeated pattern.
Similarly, U.S. Pat. No. 4,189,209 (Patent Document 8) by Heasley also describes a retroreflective article in which two kinds of hexagonal prisms having different thickness are arranged in a repeated pattern.
U.S. Pat. No. 6,114,009 (Patent Document 9) by Smith describes a mold suitable for forming cube corner retroreflective sheeting, a method for producing the mold, and retroreflective sheeting formed using the mold, and particularly discloses a mold formed of a plurality of thin laminae and a method for producing the mold.
Japanese Utility Model Application Laid-Open No. 63-109233 (Patent Document 10) by Kato describes a reflective article formed of a first reflecting part having reflective performance with respect to an incident light beam at a critical angle or larger from the left and a second reflecting part having reflective performance with respect to an incident light beam at a critical angle or larger from the right.
U.S. Pat. No. 6,120,280 (Patent Document 11) and U.S. Pat. No. 6,010,609 (Patent Document 12) by Mimura et al. disclose a design of hexagonal cube corner retroreflective elements having an asymmetrical shape in which optical axes are tilted leftward and rightward of the elements and a method of making the element.
It is described therein that entrance angle characteristics are improved in two directions, in which the optical axes are tilted, according to such hexagonal cube corner retroreflective elements having the optical axes tilted to the right and left of the elements. However, in the elements disclosed in these patent documents, excellent improvement cannot be achieved in rotation angle characteristics.
On the other hand, U.S. Pat. No. 6,318,866 (Patent Document 13) by Mimura et al. discloses various proposals on improvement of observation angles.
Patent Document 13 discloses that observation angle characteristics can be improved by forming opposite lateral faces of a pair of triangular pyramidal cube corner retroreflective elements in different shapes.
U.S. Pat. No. 4,775,219 (Patent Document 14) by Appeldorn et al. discloses a method of improving the observation angle characteristics of a retroreflective article formed of a group of triangular pyramidal retroreflective elements.
The method of improving the observation angle disclosed in Patent Document 14 is that the angle of a V-shaped groove forming the element in three directions is changed in repeating patterns in lateral asymmetry at an angle different from the angle of the adjacent V-shaped groove, whereby forming triangular pyramidal retroreflective elements with various prism apexes.